1. Field of the Invention
This invention relates to devices for determining the magnetostrictive characteristics of a test specimen.
2. Description of the Prior Art
Magnetostriction is a phenomenon exhibited by certain ferromagnetic metals such as nickel, iron, cobalt, and manganese alloys, whereby the metal expands or contracts when exposed to a magnetic field. Electrical devices which operate by electromagnetic induction, such as transformers, are constructed of these ferromagnetic metals.
In a transformer, the core is constructed of a ferromagnetic metal having primary and secondary windings thereon. During operation, an alternating magnetic field is induced in the core by an alternating electric current applied to the windings. The frequency of the alternating magnetic field is equal to the frequency of the AC electric current applied to the windings. The alternating magnetic field causes the core of the transformer to expand and contract according to the polarity of the magnetic field. Thus, the cycle of expansion and contraction has a frequency equal to the frequency of the alternating magnetic field. The amount of expansion or contraction, which is the magnetostrictive movement, is related to the strength of the alternating magnetic field. The magnetostrictive movement is responsible for losses in the core of the transformer and is responsible for producing an audible noise or hum which may have deleterious effects on personnel continually working in the area. It is therefore advantageous to measure the magnetostrictive characteristics of a metal in order to minimize core losses and noise.
In U.S. Pat. No. 2,312,888, magnetostriction is measured by an apparatus utilizing mechanical levers and a monochromatic light source. A sample is supported under a desired tension in a vertical direction between a stationary support and a weight. A coil is provided for magnetizing the sample, the coil being supported so as to avoid contact with the sample. The weighted end of the sample is pivotally connected to a first end of a horizontal lever. The second end of the horizontal lever is pivotally connected through a vertical dummy sample to a second horizontal lever. The dummy sample is substantially the same as the sample to be tested. A second non-inductive coil is provided for heating the dummy sample. The second horizontal lever is pivotally connected at one end to the dummy sample and is connected at the other end to a transparent optically flat plate having a partially reflecting under-surface. A second and stationary optically flat plate having an upper reflecting surface is located beneath the first plate. A monochromatic light source is located above the plates. Since the two plates are located relatively close together, an interference pattern will be produced. The first horizontal lever is pivoted at its center so that elongations due to equal heating of the sample and the dummy sample will not cause any movement of the second horizontal lever. The movement conveyed to the second horizontal lever will therefore be that caused only by magnetostriction and not heating of the sample. As the second horizontal lever moves, it changes the location of the first plate relative to the second plate thus changing the interference pattern. By analysis of the interference pattern, the magnetostriction of the sample may be determined.
In U.S. Pat. No. 2,596,752, a test sample is vertically mounted at one end to a stationary horizontal test fixture. The second end of the test sample is connected to a movable grating. A dummy sample is vertically mounted at one end to the stationary horizontal test fixture. The second end of the dummy sample is connected to a fixed grating. The test sample is provided with damping weights to minimize the possibility of mechanical resonance and is provided with an inductive coil for magnetizing the sample. The dummy sample is provided with a non-inductive coil for heating the dummy sample. The fixed grating and the movable grating are positioned intermediate a light source and a photosensitive device. Expansion or contraction of the test sample produces a consequent displacement of the movable grating relative to the fixed grating thereby proportionally altering the amount of light reaching the photosensitive device through the gratings. Temperature changes accompanying the magnetization of the test sample are compensated by heating the dummy sample to the same temperature as the test sample such that the position of the fixed grating relative to the movable grating remains unchanged. Thus, any movement of the test sample relative to the dummy sample is due to magnetostriction and not heating. Analysis of the amount of light reaching the photosensitive device provides an indication of the magnetostriction of the test sample.
In U.S. Pat. No. 2,850,697, a variable capacitor is used to measure magnetostriction. A test sample is vertically suspended inside a coil by a clamping means. A first end of the test sample has a plain surface which cooperates with a fixed plate to provide a variable capacitor. The test sample is clamped at a point remote from that end of the sample which is acting as a plate of the variable capacitor. For test samples of small mass, the clamping point may be the opposite end of the test sample. For test samples of large mass, the clamping point should be a natural node region. The clamping means also provides an electrical connection between the test sample and ground. When the test sample is subjected to a magnetic field, its length will change thus changing the value of the variable capacitor. An analysis of the value of the variable capacitor will provide an indication of the magnetostriction of the test sample.
In U.S. Pat. No. 4,041,377, a displacement sensitive transducer is mechanically associated with a test sample for generating a magnetostriction signal reflecting the magnetostrictive deformation of the test sample in response to an induced magnetic field. The invention includes a timing circuit associated with the means for inducing the magnetic field in the test specimen. The timing circuit produces a series of timing signals identifying the occurrence of zero and maximum induction in the test sample. A conversion circuit operates under the control of the timing circuit for deriving a DC difference signal from the magnetostriction signal produced by the displacement transducer. The DC difference signal is proportional to the difference between the amplitudes of the magnetostriction signal corresponding to the occurrences of maximum and zero induction in the test sample. The DC difference signal, which reflects the magnetostrictive characteristics of the test sample, is applied to a display which produces a digital output corresponding to the magnitude and polarity of the DC difference signal.
Unique problems are encountered in attempting to measure magnetostriction since the movement to be measured is typically on the order of 10.sup.-7 meters per meter and a test sample may be only a few mils thick. Since the movement is so small and the test sample is not structurally rigid, the use of the four patents described above, which are representative of the prior art, is subject to certain limitations. Devices which relay upon a system of mechanical levers for measuring magnetostriction are not suitable for measuring periodic or dynamic magnetostriction due to elasticity mechanical resonance and inertia of the levers. Devices which have elements, such as a grating, suspended from the test sample, or which utilize damping weights will introduce undesirable strain in the test sample which may alter the sample's magnetostrictive characteristics. Devices which utilize mechanical clamps for holding the test sample or which have transducers mechanically connected to the test sample may restrict the movement of the test sample or may unduly complicate the positioning of the sample in the testing apparatus. The present invention is for an apparatus for measuring magnetostriction which overcomes these and other limitations of the prior art.